THE PIGEON 163 



mals, the other group of warm-blooded vertebrates,— is removed 

 to the gizzard, a part of the stomach, which is situated very near 

 the center of gravity. 



Another effect which has been correlated with the loss of teeth 

 in the bird is the development of a greater intelligence. Inasmuch 

 as the weight of the head is strictly limited by the conditions of 

 the animal's existence, a larger brain could develop than would 

 have been possible if the teeth, which characterized primitive 

 birds, had not disappeared. 



The long neck is loosely inclosed in the skin. Near the base of 

 the neck is the crop, an expansion of the oesophagus for the stor- 

 age of food ; in it, also, is secreted the milklike substance called 

 pigeon's milk, with which the nestlings are fed. 



The trunk is made up of two subregions,— the thoracic and the 

 abdominal region. To the former belong the wings ; to the latter 

 the legs, by means of which the bird exercises its two very differ- 

 ent methods of locomotion. Both these subregions are rigid ones, 

 inasmuch as the attachment of the locomotory organs to the 

 trunk must be firm and solid; thus the whole trunk is a firm, 

 boxlike structure. The viscera in the trunk are very compactly 

 arranged, much more so than in the mammal, and are supported 

 in the troughlike dorsal surface of the breastbone. On the ventral 

 side of the thoracic region, on each side of the breastbone,— the 

 keel of which can be felt in the median line,— are the great pec- 

 toral muscles, the principal muscles of flight. In some birds, as 

 the humming bird, these muscles constitute more than half the 

 bulk of the body; in the pigeon they form about a fourth. 



The trunk contains the air sacs, which are large, thin-walled 

 sacs lying among the other viscera, and communicating with 

 the bronchi of the lungs and with the hollow centers of many of the 

 bones. These sacs are reservoirs of air ; their function is some- 

 what obscure, but they probably help to supply the lungs during 

 rapid flight when sufficient air could not be inspired through 

 the nostrils to supply the demands of the system. Place a blow- 

 pipe in the glottis, at the root of the tongue, and blow the lungs 

 full of air ; it will be seen that the entire trunk swells out, all the 

 air sacs being filled. 



